The invention relates to a motor vehicle body with stiffening struts, in particular a body of a cabriolet vehicle.
Diagonal struts are used on the under body for the purpose of stiffening motor vehicle bodies, in particular on cabriolet vehicles. Generally four diagonal metallic struts in cruciform arrangement are fixed on the under body of the vehicle, in order to almost double the stiffness of the vehicle body. As is known, diagonal struts, which offer a maximum stiffness, are made of steel; however steel is disadvantageous in the sense of achieving a lightweight form of construction of the vehicle body and in terms of fuel economy.
German patent document DE 10 2009 053 979 A1 discloses a diagonal strut arrangement under the floor of the vehicle, which confers improved stiffness on the vehicle body. One end of each diagonal strut is fixed to the side skirting of the vehicle body and the other end is fixed in a hollow, which is arranged in the storage space area of the vehicle. This hollow, which for example is used as the spare wheel well, can be manufactured from a fiber-reinforced plastic for the purpose of weight reduction.
In order to extend the possibilities of a strut geometry and connection to the vehicle body, German patent document DE 10 2004 018 977 A1 describes a motor vehicle with a body, whose under body region is arranged with at least two stiffening struts, which respectively extend as a component from a body connection point to a vertical longitudinal median plane, at which point a connection frame is provided. This connection frame is freely arranged below the under body and at a distance from the latter; the struts being attached to this frame. Finally, German patent document DE 10 2008 056 146 A1 describes a molding or cladding for an under floor component of a motor vehicle, which is designed to increase the resistance and the stiffening combating the torsion loadings, both at a standstill and during the motion of the vehicle. This cladding is intended to be simple, favorably-priced and stable. To this end the cladding is formed from a fiber-reinforced plastic, in particular a sheet moulding compound.
Exemplary embodiments of the present invention are directed to the use of struts to provide a stiffened motor vehicle body, with a reduced weight while at the same time maximum stiffness is retained, so that the fuel consumption of the vehicle can be reduced.
In accordance with a first exemplary embodiment of the motor vehicle body according to the invention stiffening struts made of fiber-reinforced plastic (FRP) material are provided, in which each stiffening strut is fixed at both ends to the respective adjoining body component has, according to the invention, an essentially unidirectional orientation of the FRP fibers in the direction of loading of the FRP stiffening strut. In addition the cross sectional area of the stiffening strut remains essentially the same along the length of the strut; a deviation of the cross sectional area being possible at the sections of the points of fixing. Accordingly, the stiffening struts have a very high stiffness despite their light method of construction and are therefore suitable for use in the stiffening of cabriolet vehicle bodies, even though special high demands are made in respect of the stability of bodies in the cabriolet type of construction.
Thus, for a given motor vehicle body the cross sectional area of the FRP stiffening strut is selected such that the product of the cross sectional area times the effective modulus of elasticity of the FRP stiffening strut corresponds to a product given by a cross sectional area of a steel stiffening strut and its effective modulus of elasticity for the prescribed motor vehicle body, so that the FRP stiffening strut of the vehicle body according to the invention provides the same stiffness as that of a corresponding steel strut. In so doing the selected cross sectional area of the FRP stiffening strut has to be larger than would be required for the corresponding steel strut. The increase in cross sectional area of the FRP stiffening strut should be at least 20%, preferably at least 25%, and particularly preferred increase of 30% based on the cross section of the corresponding steel stiffening strut.
Thus, it is possible to stiffen the motor vehicle body with stiffening struts made from fiber-reinforced material at least to an extent achieved using steel struts, as in the prior art. Accordingly, the form of strut according to the invention provides an advantageous weight saving of up to 60% when a steel strut suitable for a vehicle body is replaced by a strut made from fiber-reinforced material.
The stiffening struts can be encased in a protective layer against mechanical damage, for example stone-pitting.
This protective layer could be, for example, a non-woven material or a wound fiber material, in particular a wound glass fiber material. An alternative protective layer can take the form of a non-abrasive, impact-resistant material, similar to a steel mesh or steel foil, however, it is also conceivable that a thermoplastic or elastomeric material could be used as a protective layer, which in particular could take the form of shrink tubing applied on the stiffening strut.
The wound fibers of a protective layer can be arranged to improve the stability and the protective effect with an angular offset with respect to the essentially unidirectional fibers of the stiffening strut.
Preferably, the stiffening strut is a pultruded fiber-reinforced/composite fiber material with a fiber content in the range between 50% and 75%, based on the overall volume of the fiber-reinforced material, preferably of 65%, and particularly preferred at a level of 70% with the attendant conferred stiffness.
The composite fiber material for the stiffening strut, with the unidirectionally-aligned reinforcing fibers, can take the form of carbon fibers in a duroplastic or thermoplastic matrix, however it is also conceivable that a mixed method of construction could be employed involving a hybrid composite fiber material with a light metal alloy matrix. In general it is also conceivable that the composite fiber material could contain a proportion of metal fibers or a metal mesh.
It is also possible that a material hybridization could be employed, for example with a metal core.
The joining concept of the stiffening struts according to the invention, before fastening to the motor vehicle body, provides that the connecting sections of the stiffening struts respectively have a joining location with respectively at least one access opening, in which a connection element, such as a screw, can be accepted.
The access opening can be machined out after the completion of the stiffening strut; however the access opening can also be provided as a cut-out in the manufacturing process. Further, the access opening can be provided by a step in the manufacturing process or by a subsequently inserted step.
Further, it is advantageous for the transmission of force in the FRP stiffening strut if the access opening is surrounded by a fiber reinforced area. Accordingly, the fibers can be arranged concentrically in rings, tangentially and/or radially around the access opening, similar to the shape of a spider's web or a target.
The complete connection section or at least the joint position can be equipped with a strengthened structure. This can be, for example, the inner lying metal core, the insert or a metal cladding of the joint position or of the connection section. This reinforcement structure can be incorporated in the stiffening strut during the manufacturing process, such that the reinforcement structure is manufactured integral with the stiffening strut. It can also be subsequently attached with adhesive, so that there is then an adhesive layer between the reinforcement structure and the stiffening strut. The adhesive layer combats corrosion of the metallic connection location on the vehicle body, in particular if the stiffening strut is made of CFRP.
A motor vehicle body being stiffened by means of the stiffening strut according to the invention can be, in particular, a cabriolet body.
These and other advantages are explained in the following description with reference to the accompanying figures. The reference to the figures in the description serves to support the description and the simplified understanding of the component parts. Components or parts of components, which are essentially the same or similar may be indicated using the same reference symbol. The figures are merely a schematic representation of an embodiment of the invention.